Let’s assume that you have already used ACID to find your camera’s firmware and then download the appropriate CHDK build. Now we are going to use SDMInst to format your sd card and then move the CHDK build onto it.
If you are running Mountain Lion (10.8) then go here in order to figure out how to start up the app. Other versions can simply double click it.
First, select your memory card from the “Select card drive:” dialog. You will know which one it is by its size in comparison to the others, and the name in the “SDM Volume:” field. You may not even have any choices. I happened to have a lot of mounted disk images that showed up as options. Next click on the browse button, or drag the directory containing your CHDK build into the “Copy all files from” box.
Finally click on “Prepare Card” and watch it do its magic. Eject the card, lock it, and then put it in your camera to test!